Roller subs or skates are downhole devices which include wheels which contact a wall of the well bore to assist in moving the device and more particularly, the tool string to which it is attached, through a well bore. Typically tool strings pass through the well bore under the influence of gravity. However, when the well bore is deviated, particularly towards the horizontal, gravity causes the string to fall to the lower side of the well bore. In this location the string can stick and the tools thereon can become damaged. This problem is exacerbated for wireline tool strings. Roller subs maintain the tool string away from the well bore wall while the wheels reduce friction between the two and thus aid in moving the tool string through the well bore.
The earliest roller subs comprise wheels located in slots around the circumference of a cylindrical body. Multiple wheels are required to prevent the body contacting the well bore wall and as a result the wheels each have a diameter approximately half that of the body.
Later designs provide larger wheels through the centre of the body. Typically there are two pairs of two wheels, with the same orientation, located through slots in the body. In order to ensure the wheels contact the lower wall of the well bore the body is weighted on one side and this, in combination with a swivel between the body and the string, causes the wheels to lie against the low side of the well bore. Alternatively a body having an elliptical or oval cross-section, with each pair of wheels mounted on the short axis as the wheels run on the longer dimension, will preferentially tip the body onto the longer dimension. This roller sub will therefore orient itself to run on the low side of the well bore.
A major disadvantage of the prior art roller subs is that the wheels are located within or through slots in the body. The wheels must therefore be of a size which fits within the slot but which protrudes sufficiently from the body to contact the well bore wall. As a result each body is designed for a single wheel diameter and thus a different roller sub is required for efficient operation in well bores of different diameters.
An object of at least one embodiment of the present invention is to provide a roller sub system whose wheel diameter can be varied.
A further object of at least one embodiment of the present invention is to provide a roller sub body to which roller wheels of variable diameters can be used.